Quantum cryptography facilitates a quantifiable secure communication. By the transmission of quantum particles, particularly photons, an arbitrary, secure key can be generated. This key can then be used for the enciphering of corresponding methods (e.g., one-time-pad, DES). Possible eavesdropping attempts change the quantum particles in a way that errors in the generated key show the attack. This is in considerable contrast to the conventional system in which the security of the transmission is based, e.g., on the faith in couriers or on (unproved) assumptions of the technological abilities of the eavesdropping person.
Quantum cryptography was theoretically suggested for the first time in 1984 and experimentally realized in 1991. Several theoretical and experimental publications and patents led to a rapid development in this field. At present, the research in the field of quantum cryptography concentrates over all on the technical implementation of first prototypes. Here great attention is paid to the miniaturization of the systems used as well as on a high stability and economic efficiency of the transmitter and receiver optics.
The prior art relevant for the present invention is documented, e.g., by:    [1] U.S. Pat. No. 5,307,410 Interferometric quantum cryptographic key distribution system CH. H. Bennett    [2] U.S. Pat. No. 5,732,139 Quantum cryptographic system with reduced data loss H-K. Lo, H. F. Chau    [3] EP 0 776 558 Quantum cryptography P. D. Townsend    [4] U.S. Pat. No. 5,243,649 Apparatus and method for quantum mechanical encryption for the transmission of secure communication J. D. Franson    [5] EP 0 923 828 Quantum cryptography device and method N. Gisin, A. Mueller, B. Perny, H. Zbinden, B. Huttner    [6] EP 0 717 895 B1 Key distribution in a multiple access network using quantum cryptography P. D. Townsend, D. W. Smith    [7] U.S. Pat. No. 5,757,912 System and method for quantum cryptography K. J. Blow    [8] EP 0 722 640 B1 Cryptographic receiver J. G. Rarity, P. R. Tapster    [9] Towards practical quantum cryptography S. Chiangga, P. Zarda, T. Jennewein, H. Weinfurther Appl. Phys. B. 69, 389 (1999)    [10] Daylight quantum key distribution over 1.6 km W. T. Buttler, R. J. Hughes, S. K. Lamoreaux, G. L. Morgan, J. E. Nordholt, C. G. Peterson Phys. Rev. lett. 84, 5652 (2000)    [11] Long distance entanglement based quantum key distribution G. Ribordy, J. Brendel, J.-D. Gautier, N. Gisin, H. Zbinden Phys. Rev. A 63, 012309 (2001)
A summarizing documentation can also be found in the article “Quantenkryptographie” by U. Gebranzig, W. Süβmuth, Jahrbuch des deutschen Patentamts 1999.
The present invention relates to a system for the secure distribution of cryptographic keys according to the method of the quantum cryptography. In the quantum cryptography, as known from U.S. Pat. No. 5,307,410 [1] and U.S. Pat. No. 5,732,139 [2], a cryptographic key is generated by transmitting information carrying light signals between two or more participants on the quantum channel, by measuring said signals and by exchanging information on the measured values via a conventional communication channel. Possible eavesdropping attacks during the transmission of the light signals can be detected according to conclusions of the quantum theory. The generated cryptographic key is of great importance for the transmission of all kinds of information due to its high security.
From [3]-[5], apparatuses are known, which are suitable for the key distribution in accordance with the principles of quantum cryptography. In particular, transmitter and receiver are described, which provide rapid switches for changing quantum mechanical states of the light signals transmitted by a signal source and detected by an analysis channel. Here, disadvantages are the high costs and the high technical efforts being necessary for the rapid operation of the switches.
According to the patents [6] and [7], the efforts can be reduced. A first simplification is described in EP 0 717 895 B1 [6]. Here apparatuses for quantum cryptography are described, in which non-orthogonal quantum states are coupled by an optical switch into the output of the transmission unit. U.S. Pat. No. 5,757,912 [7] describes a method in which 2 sources generate orthogonal light states. Thus, a reduction in the efforts by the factor 2 for the phase modulation is achieved.
From documents [8]-[11], apparatuses are known in which no switches are necessary due to the use of 2 or more signal sources or 2 or more analysis channels. Here, the light signals emitted by the signal sources are superimposed in the transmitter device by means of optical components, especially semipermeable mirrors, or the light signals are split in the receiver device by means of a semipermeable mirror. Here, the signal sources are controlled in such a way that only one of the sources generates at a time one single photon or a reduced light impulse at the output of the transmitter device. According to patent EP 0 722 640 B1 [8], in the receiver the incoming photon is distributed at random on the different analyzers by a beam splitter and is registered by one of the detectors, thus forming the signal relevant for the key generation.
During the technical realization, it has proven especially disadvantageous that therefor the direction of the signal sources has to be accurately adjusted and that necessary components, particularly the semipermeable mirrors, change the quantum mechanical state in an undesired manner. For its correction, further optical components have to be introduced and to be adjusted accurately. The high number of optical components and the high adjusting efforts cause an increased space requirement, a non-optimal signal/noise ration and a bad stability of the systems. It has also proven disadvantageous that especially for the use of new, more efficient methods (U.S. Pat. No. 5,732,139 [2]), a variation of the splitting ration can only be achieved by exchanging and adjusting optical components anew.
The present invention is based on the object to provide an improved apparatus and an improved method for the quantum cryptography.
This object is solved with the subject-matter of the claims.